Field
One or more example embodiments relate to a method of preparing conductive single crystal silicon particles coated with highly conductive carbon containing nanopores and an ultrathin metal film and an anode material for the secondary battery using the same. More specifically, one or more example embodiments relate to conductive single crystal silicon particles coated with highly conductive carbon containing nanopores and an ultrathin metal film, a high capacity anode material for the secondary battery including the same, and a preparing method thereof.
Description of Related Art
A lithium (Li) secondary battery has no memory effect while showing a high energy density, power density, and a low self-discharge rate. In this regard, the lithium the secondary battery has been widely used as an electrical energy storage device of portable electronic devices. In the recent times, with the introduction of electric vehicles, the lithium the secondary battery has been employed for further various purposes. However, the lithium the secondary battery using graphite anode has the low capacity, it may be difficult to design a cell that is provided with a high energy density and lightness at the same time.
Materials which can replace the graphite anode may include silicon, tin, and germanium, which are elements belonging to the same group as carbon constituting graphite. Here, silicon may achieve the high capacity (3580 mA h g−1 for Li15Si4 at a room temperature), which approaches about ten times compared to that of the graphite anode. In addition, silicon has a relatively low potential response (<0.4 vs. Li/Li+) against Li. Further, since a large amount of silicon is present on the earth, silicon is significantly advantageous in terms of price compared to other replacements and is nontoxic. However, during the charging, silicon accompanies relatively great volume expansion (>300%, Li3.75Si at room temperature) which leads to the delamination between active material and a collector. As a result, after a few initial cycles, the capacity decreases significantly. In addition, due to a low electrical conductivity (10−5 S/cm), a sharp capacity drop occurs in high rate charging and discharging.
Accordingly, to use silicon, which is an ultra-high capacity anode material, researches are generally on 1) mitigating the volume expansion of silicon and 2) enhancing the electrical conductivity of silicon. Initially, to mitigate the volume expansion of silicon, representative ongoing research relates to preventing silicon particles from being crushed even in the case of the volume expansion by grinding bulk silicon particles into nanoparticles and thereby reducing the internal stress of bulk silicon particles in the case of the volume expansion or by introducing a porous structure. Also, research for decreasing the volume expansion by forming a phase (SiO2, SiC, Si3N4, etc.,) not-reacting lithium with silicon particles is ongoing.
In order to enhance the electrical conductivity of silicon, research for coating the surface of silicon with a conductive material has been conducted. Research having achieved the enhanced cycles and high rate charging/discharging results by coating the surface of silicon particles with carbon showing a representatively high electrical conductivity has been published. In addition, there is ongoing research for enhancing the electrical conductivity by coating the surface of silicon particles with a metal, graphene, graphene oxide, and reduced graphene oxide. Such surface coating serves to improve the electrical conductivity and to enhance cycling performances by stabilizing an unstable silicon/electrolyte interface layer and accordingly, has become requirements to use silicon as a high capacity anode material.
Further, a method which can overcome the disadvantages of silicon such as the volume expansion and a relatively low conductivity is required to effectively use the high capacity of silicon. Accordingly, silicon may be covered with a material having a high electrical conductivity and also alleviating the volume expansion of silicon. Representatively, many researches for applying conductive carbon on the surface of silicon have been conducted. However, the existing method forms a carbon coating layer on the surface of silicon by applying a chemical vapor deposition (CVD) or a physical vapor deposition under high temperature using hydrocarbon gas (CH4, C2H4, C7H8, etc.) that is explosive and harmful to a body under high temperature or high pressure. The method is dangerous and also requires high-cost facility and process, which makes a mass production difficult.
The most appropriate technology for commercializing a silicon anode is a process method that, without a complex additional process during an existing process of applying a conductive carbon coating layer to silicon particles, may easily and in large quantity produce silicon particles coated with a carbon coating layer which can mitigate the volume expansion of silicon, and containing pore layers which can reduce a stress occurring due to the volume expansion of silicon particles.